A power supply is used to convert electric power from one form to another to suit the particular load in the system. Generally, there is a strong desire to operate the power supply at a high switching frequency to reduce the size and improve the performance of the power supply. A high frequency operation is especially important for applications in wireless power transfer, where higher frequency helps also to transfer more power over longer distance. For example, many wireless power transfer systems operate at 6.78 MHZ or 13.57 MHZ.
To reduce the power loss of power switches at high switching frequency, resonant power converters were developed to improve the efficiency of the power conversion. However, a resonant power converter can achieve the best efficiency only when it operates at or close to a resonant frequency. Because of the component tolerance of the resonant tank, the resonant frequency cannot be easily set to a particular value, so it is difficult to maintain high efficiency operation of a resonant converter.
Also, in many wireless power transfer systems, there is a need to control the voltage and power at the receiver. Currently a separate power stage is used to regulate the power transfer. Such a system incurs high cost and suffers from low efficiency.